In systems where long-distance, large-capacity transmission is carried out over optical fibers, low-cost achievement of high-speed, high-density signal demultiplexing and low-cost reduction of performance degradation associated with the transmission are challenging.
Transmission capacity per optical fiber can be increased if an optical transmission apparatus that transmits optical signals carries out dense wavelength multiplexing in order to make a plurality of optical carrier waves or optical sub-carrier waves, which are sub-carriers, carry different information items. The optical carrier waves and the optical sub-carrier waves that undergo multiplexing here are referred to as channels. Furthermore, the transmission capacity can also be increased even by using multi-level modulation.
Conceivable methods for achieving long-distance transmission and large-capacity transmission include using m-ary phase-shift keying (mPSK) and using m-ary quadrature amplitude modulation (mQAM). In other words, an increased number of constellation points for an increased number of transmission bits per symbol enables increased transmission capacity. In mPSK and mQAM, a signal is typically assigned to an in-phase axis (I axis) and a quadrature-phase axis (Q axis).
Polarization multiplexing is a known method of doubling the number of transmission bits per symbol. With polarization multiplexing, a signal can be assigned to two independent orthogonal polarization components, namely, a vertically polarized wave and a horizontally polarized wave.
In an optical transmission system where polarization multiplexing mPSK or polarization multiplexing mQAM is used, a receiving end uses a digital coherent method in which mixed interference is caused between a receive signal and continuous light produced by a local oscillation light source for coherent detection, and the electrical signal obtained by the coherent detection undergoes digital signal processing for compensation. In the digital coherent method, dual-polarization (DP) quadrature PSK (QPSK) is widely used (refer to, for example, Non-Patent Literatures 1 and 2).
In mPSK and mQAM, m is typically two raised to the power n (n: an integer greater than or equal to one), and thus n-bit information communication is enabled. Meanwhile, restricting typical signal constellations in mPSK and mQAM to improve the performance is code modulation that is being examined (refer to, for example, Non-Patent Literatures 3 and 4). In the simplest conceivable example, bits for communication in one block are (n−1) in number, and constellation points are prepared for the n bits in that one block. The (n−1) bits for communication are exclusive-ORed for generation of one parity bit to carry out communication using constellation points for n bits. Working on, for example, two orthogonal polarizations, two orthogonal phases, and a plurality of time slots all together is a typical method of constructing a block. N-dimensional modulation is the name used when coordinate axes are N in number. N is an integer greater than or equal to one. The N-dimensional modulation has a limited improvement in performance compared with an error correction code that has a long code length. However, the N-dimensional modulation is advantageous in that the number of information bits and the number of parity bits can be varied in relation to each other, so that spectral efficiency can be flexibly varied. For example, polarization switched-QPSK (PS-QPSK) that achieves three bits per symbol is a predominant intermediate solution between typical DP-QPSK that achieves four bits per symbol and DP-binary phase-shift keying (DP-BPSK) that achieves two bits per symbol.
In long-distance optical transmission, it is necessary to achieve the optical signal-to-noise ratio that corresponds to, for example, a bit rate, modulation, and a detection method in order to ensure the signal quality at a receiving end. To this end, signal transmission requires high optical power, and when this is the case, waveform distortion ascribable to nonlinear optical effects in the optical fiber causes degraded signal quality. The nonlinear optical effects can be classified broadly into effects that take place in a channel and effects that take place between channels.
Self-phase modulation (SPM) is one example a nonlinear optical effect that takes place in a channel. In narrower definitions, SPM is classified as intra-channel SPM (ISPM), intra-channel cross-phase modulation (IXPM), intra-channel four-wave mixing (IFWM), or the like. Furthermore, nonlinear optical effects that take place between channels include cross-phase modulation (XPM), four-wave mixing (FWM), and cross-polarization modulation (XPolM). The nonlinear optical effects more markedly take place in a channel and between channels when a signal is of high optical power density, when the optical power density of a signal varies greatly, and when the transmission distance is long. Moreover, in the case of the nonlinear optical effects that take place between channels, there is a long correlation between polarization states of respective optical signals of the channels in a transmission line when local chromatic dispersion is small in the transmission line or when the wavelength spacing between channels that undergo wavelength multiplexing is small. Continued interaction leads to markedly degraded quality.
By restricting the possible combination of constellation points, code modulation reduces power variation in optical signals and thus can also improve tolerance to the nonlinear optical effects. In the 4-dimensional 2-ary amplitude 8-ary PSK (4D-2A8PSK) described in Patent Literature 1, for example, four dimensions formed by two orthogonal polarizations, two orthogonal phases, and one time slot form one block, and each of the polarizations has a 2-ary amplitude 8-ary PSK (2A8PSK) signal constellation. With 4D-2A8PSK, bits used communication in one block are six in number, and constellation points are 256 in number for eight bits in that one block. FIG. 13 illustrates bit mapping and symbol mapping for 4D-2A8PSK. The left side of FIG. 13 illustrates a 4D-2A8PSK signal constellation in an X-polarization plane as well as in a Y-polarization plane, and the correspondence between four bits to transmit and each of the constellation points. Each of the three bits on the left side of the comma are information bits, while a single bit on the right side of the comma is a parity bit. As illustrated in FIG. 13, in communication using 4D-2A8PSK, of the six information bits for communication, three bits are Gray-coded and are assigned to the phase of an X-polarized wave, and the other three bits are Gray-coded similarly and are assigned to the phase of a Y-polarized wave. Moreover, these six bits are exclusive-ORed to generate one parity bit that is assigned to the amplitude of the X-polarized wave, and a second parity bit is obtained by inversion of the first parity bit assigned to the amplitude of the X-polarized wave and is assigned to the amplitude of the Y-polarized wave. In this way, with 4D-2A8PSK, the eight bits in total are transmitted in one block. As such, optical signal power per time slot is constant with respect to any bit combination. Although 4D-2A8PSK that achieves the constant optical signal power can achieve a spectral efficiency of six bits per symbol, which is the same as that of DP-8QAM, 4D-2A8PSK has high tolerance to the nonlinear optical effects compared with DP-8QAM with which optical signal power varies depending on bit combination. The 4D-2A8PSK that allows transmission of six information bits in one block is hereinafter referred to as 6b4D-2A8PSK.